Proton-conducting membrane and use thereof

ABSTRACT

The present invention relates to a novel proton-conducting polymer membrane based on polyazoles which can, owing to its excellent chemical and thermal properties, be used for a variety of purposes and is particularly suitable as a polymer-electrolyte membrane (PEM) for the production of membrane electrode units for so-called PEM fuel cells.

RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. 371)of PCT/EP2004/014830 filed Dec. 30, 2004, which claims benefit Germanpatent application no. 103 61 833.5 filed Dec. 30, 2003.

The present invention relates to a novel proton-conducting polymermembrane based on polyazoles which can, owing to its excellent chemicaland thermal properties, be used for a variety of purposes and isparticularly suitable as a polymer-electrolyte membrane (PEM) inso-called PEM fuel cells.

Polyazoles such as polybenzimidazoles (®Celazole) have been known for along time. The production of such polybenzimidazoles (PBI) is usuallyconducted by reacting 3,3′,4,4′-tetraaminobiphenyl with isophthalic acidor its esters in the melt. The resulting prepolymer solidifies in thereactor und is subsequently comminuted mechanically. The pulverulentprepolymer is then fully polymerised in a solid-state polymerisation attemperatures of up to 400° C. and the desired polybenzimidazole isobtained.

To produce polymer films, the PBI is dissolved in an additional step inpolar, aprotic solvents such as dimethylacetamide (DMAc) and a film isproduced by means of classical processes.

Proton-conducting, i.e. acid-doped polyazole membranes for use in PEMfuel cells are already known. The alkaline polyazole films are dopedwith concentrated phosphoric acid or sulphuric acid and then act asproton conductors and separators in so-called polymer electrolytemembrane fuel cells (PEM fuel cells).

Due to the excellent properties of the polyazole polymer, such polymerelectrolyte membranes—processed to produce membrane electrode units(MEUs)—can be employed in fuel cells at long-term operating temperaturesabove 100° C., in particular above 120° C. This high long-term operatingtemperature allows increasing the activity of the catalysts based onnoble metals which are included in the membrane electrode unit (MEU).Especially when the so-called reformates from hydrocarbons are used, thereformer gas contains considerable amounts of carbon monoxide whichusually have to be removed by means of an elaborate gas conditioning orgas purification process. Owing to the possibility to increase theoperating temperature, significantly higher concentrations of COimpurities can be tolerated permanently.

By employing polymer electrolyte membranes based on polyazole polymers,it is possible, on the one hand, to partly dispense with the elaborategas conditioning or gas purification process and, on the other hand, toreduce the catalyst load in the membrane electrode unit. These are bothindispensable preconditions for a large-scale use of PEM fuel cells asotherwise the cost for a PEM fuel cell system is too high.

The previously known acid-doped polymer membranes based on polyazolesalready display a beneficial property profile. However, due to theintended applications for PEM fuel cells, in particular in theautomobile sector and the decentralised electricity and heat generation(stationary sector), these need to be improved altogether. Furthermore,the previously known polymer membranes have a high content ofdimethylacetamide (DMAc) which cannot be removed completely by means ofknown drying methods. The German patent application No. 10109829.4describes a polymer membrane based on polyazoles in which the DMAccontamination was removed.

Polymer membranes based on polyazoles which are produced frompolyphosphoric acids are known from the German patent applications No.10117686.4, 10117687.2 and 10144815.5. These membranes display anexcellent performance, in particular at operating temperatures above100° C. However, these membranes have the disadvantage that they exhibita relatively high overvoltage, in particular at the cathode.

It is an object of the present invention to provide polymer membranesthat are based on polyazoles and contain organic acid, which, on the onehand, display the advantages of the polymer membrane based on polyazolesin terms of application technology and, on the other hand, have anincreased specific conductivity, in particular at operating temperaturesabove 100° C., and additionally exhibit a markedly lower overvoltage, inparticular at the cathode.

We have now found that a proton-conducting membrane based on polyazolescan be obtained when the commercially available polyazole polymer issuspended or dissolved in organic phosphonic anhydrides resp. dissolvedfor the purpose of producing the membrane.

With this new membrane, it is possible to dispense with the specificpost-treatment described in the German Patent application No.10109829.4.

The object of the present invention is a proton-conducting polymermembrane based on polyazoles which can be obtained by a processcomprising the steps of

-   A) dissolving the polyazol-polymer in organic phosphonic anhydrides    with formation of a solution and/or dispersion,-   B) heating the solution obtainable in accordance with step A) under    inert gas to temperatures of up to 400° C., preferably up to 350°    C., particularly of up to 300° C.,-   C) forming a membrane using the solution of the polyazole polymer in    accordance with step B) on a support and-   D) treatment of the membrane formed in step C) until it is    self-supporting.

The polymers used in step A) based on polyazole contain recurring azoleunits of the general formula (I) and/or (ii),

wherein

-   Ar are identical or different and represent a tetracovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar¹ are identical or different and represent a bicovalent aromatic    or heteroaromatic group which can be mononuclear or polynuclear,-   Ar² are identical or different and represent a bicovalent or    tricovalent aromatic or heteroaromatic group which can be    mononuclear or polynuclear,-   X are identical or different and represent oxygen, sulphur or an    amino group which carries a hydrogen atom, a group having 1-20    carbon atoms, preferably a branched or unbranched alkyl or alkoxy    group, or an aryl group as a further radical,

Preferred aromatic or heteroaromatic groups are derived from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenylsulphone, quinoline,pyridine, bipyridine, anthracene and phenanthrene, which optionally alsocan be substituted.

In this case, the substitution pattern of Ar¹ can be in any form; in thecase of phenylene, for example, Ar¹ can be ortho-, meta- andpara-phenylene. Particularly preferred groups are derived from benzeneand biphenylene which optionally also can be substituted.

Preferred alkyl groups are short-chain alkyl groups having 1 to 4 carbonatoms, e.g. methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups can be substituted.

Preferred substituents are halogen atoms, e.g. fluorine, amino groups orshort-chain alkyl groups, e.g. methyl or ethyl groups.

Polyazoles having recurring units of the formula (I) are preferred wherethe radicals X within one recurring unit are identical.

The polyazoles can in principle also have different recurring unitswhere their radicals X are different, for example. It is preferable,however, that a recurring unit has only identical radicals X.

In another embodiment of the present invention, the polymer containingrecurring azole units is a copolymer, which contains at least two unitsof the formula (I) and/or (II), which differ from one another. Thepolymers can be in the form of block copolymers (diblock, triblock),random copolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer containing recurring azole units is a polyazole which onlycontains units of the formulae (I) and/or (II).

The number of recurring azole units in the polymer is preferably aninteger greater than or equal to 10. Particularly preferred polymerscontain at least 100 recurring azole units.

Within the scope of the present invention, polymers containing recurringbenzimidazole units are preferred. Some examples of the most appropriatepolymers containing one or more recurring benzimidazole units arerepresented by the following formulae:

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

The polyazoles used, in particular, however, the polybenzimidazoles arecharacterized by a high molecular weight. Measured as the intrinsicviscosity, this is at least 0.2 dl/g, preferably 0.2 to 3 dl/g.

Further preferred polyazole polymers are polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

The organic phosphonic anhydrides used in step A) are cyclic compoundsof the formula

or linear compounds of the formula

or anhydrides of the multiple organic phosphonic acids, such as of theformula of anhydrides of the diphosphonic acid

wherein the radicals R and R′ are identical or different and represent aC₁-C₂₀ carbon-containing group.

Within the scope of the present invention, a C₁-C₂₀ carbon-containinggroup is understood to mean preferably the radicals C₁-C₂₀ alkyl,particularly preferably methyl, ethyl, n-propyl, i-propyl, n-butyl,i-butyl, s-butyl, t-butyl, n-pentyl, s-pentyl, cyclopentyl, n-hexyl,cyclohexyl, n-octyl or cyclooctyl C₁-C₂₀ alkenyl, particularlypreferably ethenyl, propenyl, butenyl, pentenyl, cyclopentenyl, hexenyl,cyclohexenyl, octenyl or cyclooctenyl, C₁-C₂₀ alkynyl, particularpreferably ethynyl, propynyl, butynyl, pentynyl, hexynyl or octynyl,C₆-C₂₀ aryl, particularly preferably phenyl, biphenyl, naphthyl oranthracenyl, C₁-C₂₀ fluoroalkyl, particularly preferablytrifluoromethyl, pentafluoroethyl or 2,2,2-trifluoroethyl, C₆-C₂₀ aryl,particularly preferably phenyl, biphenyl, naphthyl, anthracenyl,triphenylenyl, [1,1′;3′,1″]-terphenyl-2′-yl, binaphthyl orphenanthrenyl, C₆-C₂₀ fluoroaryl, particularly preferablytetrafluorophenyl or heptafluoronaphthyl, C₁-C₂₀ alkoxy, particularlypreferably methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy,s-butoxy or t-butoxy, C₆-C₂₀ aryloxy, particularly preferably phenoxy,naphthoxy, biphenyloxy, anthracenyloxy, phenanthrenyloxy, C₇-C₂₀arylalkyl, particularly preferably phenoxy, naphthoxy, biphenyloxy,anthracenyloxy, phenanthrenyloxy, C₇-C₂₀ arylalkyl, particularlypreferably o-tolyl, m-tolyl, p-tolyl, 2,6-dimethylphenyl,2,6-diethylphenyl, 2,6-di-i-propylphenyl, 2,6-di-t-butylphenyl,o-t-butylphenyl, m-t-butylphenyl, p-t-butylphenyl, C₇-C₂₀ alkylaryl,particularly preferably benzyl, ethylphenyl, propylphenyl,diphenylmethyl, triphenylmethyl or naphthalenylmethyl, C₇-C₂₀aryloxyalkyl, particularly preferably o-methoxyphenyl, m-phenoxymethyl,p-phenoxymethyl, C₁₂-C₂₀ aryloxyaryl, particularly preferablyp-phenoxyphenyl, C₅-C₂₀ heteroaryl, particularly preferably 2-pyridyl,3-pyridyl, 4-pyridyl, quinolinyl, isoquinolinyl, acridinyl,benzoquinolinyl or benzoisoquinolinyl, C₄-C₂₀ heterocycloalkyl,particularly preferably furyl, benzofuryl, 2-pyrrolidinyl, 2-indolyl,3-indolyl, 2,3-dihydroindolyl, C₈-C₂₀ arylalkenyl, particularlypreferably o-vinylphenyl, m-vinylphenyl, p-vinylphenyl, C₈-C₂₀arylalkynyl, particularly preferably o-ethynylphenyl, m-ethynylphenyl orp-ethynylphenyl, C₂-C₂₀ heteroatom-containing group, particularlypreferably carbonyl, benzoyl, oxybenzoyl, benzoyloxy, acetyl, acetoxy ornitril, where one or more C₁-C₂₀ carbon-containing groups can form acyclic system.

In the above-mentioned C₁-C₂₀ carbon-containing groups, one or more CH₂groups that are not adjacent to each other can be replaced by —O—, —S—,—NR¹— or —CONR²— and one or more H atoms can be replaced by F.

In the above-mentioned C₁-C₂₀ carbon-containing groups which can includethe aromatic systems, one or more CH groups that are not adjacent toeach other can be replaced by —O—, —S—, —NR¹— or —CONR²— and one or moreH atoms can be replaced by F.

The radicals R¹ and R² are identical or different at each occurrence ofH or are an aliphatic or aromatic hydrocarbon radical having 1 to 20 Catoms.

Particularly preferred are organic phosphonic anhydrides which arepartially fluorinated or perfluorinated.

The organic phosphonic anhydrides used in step A) are commerciallyavailable, for example the product ®T3P (propane phosphonic anhydride)from the company Clariant.

The organic phosphonic anhydrides used in step A) can also be employedin combination with polyphosphoric acid and/or P₂O₅. The polyphosphoricacids are customary polyphosphoric acids as they are available, forexample, from Riedel-de Haen. The polyphosphoric acidsH_(n+2)P_(n)O_(3n+1) (n>1) usually have a concentration of at least 83%,calculated as P₂O₅ (by acidimetry). Instead of a solution of themonomers, a dispersion/suspension can also be produced.

The organic phosphonic anhydrides used in step A) can also be employedin combination with single or multiple organic phosphonic acids.

The single and/or multiple organic phosphonic acids are compounds of theformula

wherein the radicals R are identical or different and represent a C₁-C₂₀carbon-containing group.

Within the scope of the present invention, a C₁-C₂₀ carbon-containinggroup is understood to mean preferably the radicals C₁-C₂₀ alkyl,particularly preferably methyl, ethyl, n-propyl, i-propyl, n-butyl,i-butyl, s-butyl, t-butyl, n-pentyl, s-pentyl, cyclopentyl, n-hexyl,cyclohexyl, n-octyl or cyclooctyl, C₆-C₂₀ aryl, particularly preferablyphenyl, biphenyl, naphthyl or anthracenyl, C₁-C₂₀ fluoroalkyl,particularly preferably trifluoromethyl, pentafluoroethyl or2,2,2-trifluoroethyl, C₆-C₂₀ aryl, particularly preferably phenyl,biphenyl, naphthyl, anthracenyl, triphenylenyl,[1,1′;3′,1″]-terphenyl-2′-yl, binaphthyl or phenanthrenyl, C₆-C₂₀fluoroaryl, particularly preferably tetrafluorophenyl orheptafluoronaphthyl, C₁-C₂₀ alkoxy, particularly preferably methoxy,ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy or t-butoxy,C₆-C₂₀ aryloxy, particularly preferably phenoxy, naphthoxy, biphenyloxy,anthracenyloxy, phenanthrenyloxy, C₇-C₂₀ arylalkyl, particularlypreferably o-tolyl, m-tolyl, p-tolyl, 2,6-dimethylphenyl,2,6-diethylphenyl, 2,6-di-i-propylphenyl, 2,6-di-t-butylphenyl,o-t-butylphenyl, m-t-butylphenyl, p-t-butylphenyl, C₇-C₂₀ alkylaryl,particularly preferably benzyl, ethylphenyl, propylphenyl,diphenylmethyl, triphenylmethyl or naphthalenylmethyl, C₇-C₂₀aryloxyalkyl, particularly preferably o-methoxyphenyl, m-phenoxymethyl,p-phenoxymethyl, C₁₂-C₂₀ aryloxyaryl, particularly preferablyp-phenoxyphenyl, C₅-C₂₀ heteroaryl, particularly preferably 2-pyridyl,3-pyridyl, 4-pyridyl, quinolinyl, isoquinolinyl, acridinyl,benzoquinolinyl or benzoisoquinolinyl, C₄-C₂₀ heterocycloalkyl,particularly preferably furyl, benzofuryl, 2-pyrrolidinyl, 2-indolyl,3-indolyl, 2,3-dihydroindolyl, C₂-C₂₀ heteroatom-containing group,particularly preferably carbonyl, benzoyl, oxybenzoyl, benzoyloxy,acetyl, acetoxy or nitril, where one or more C₁-C₂₀ carbon-containinggroups can form a cyclic system.

In the above-mentioned C₁-C₂₀ carbon-containing groups, one or more CH₂groups that are not adjacent to each other can be replaced by —O—, —S—,—NR¹— or —CONR²— and one or more H atoms can be replaced by F.

In the above-mentioned C₁-C₂₀ carbon-containing groups which can includethe aromatic systems, one or more CH groups that are not adjacent toeach other can be replaced by —O—, —S—, —NR¹— or —CONR²— and one or moreH atoms can be replaced by F.

The radicals R¹ and R² are identical or different at each occurrence ofH or are an aliphatic or aromatic hydrocarbon radical having 1 to 20 Catoms.

Particularly preferred are organic phosphonic acids which are partiallyfluorinated or perfluorinated.

The organic phosphonic acids used in step A) are commercially available,for example the products from the company Clariant or Aldrich.

The organic phosphonic acids used in step A) comprise novinyl-containing phosphonic acids as are described in the German patentapplication No. 10213540.1.

The mixture produced in step A) has a weight ratio of organic phosphonicanhydrides to the sum of all polymers of from 1:10,000 to 10,000:1,preferably 1:1000 to 1000:1, in particular 1:100 to 100:1. If thesephosphonic anhydrides are used in a mixture with polyphosphoric acid orsingle and/or multiple organic phosphonic acids, these have to beconsidered in the phosphonic anhydrides.

In addition, further organophosphonic acids, preferably perfluorinatedorganic phosphonic acids can be added to the mixture produced in stepA). This addition can take place before and/or during step B) resp.before step C). Through this, it is possible to control the viscosity.

The layer formation in accordance with step C) is performed by means ofmeasures known per se (pouring, spraying, application with a doctorblade) which are known from the prior art of polymer film production.Every support that is considered as inert under the conditions issuitable as a support. To adjust the viscosity, phosphoric acid (conc.phosphoric acid, 85%) can be added to the solution, where required.Through this, the viscosity can be adjusted to the desired value and theformation of the membrane be facilitated. The temperature of thesolution heated is up to 400° C., preferably between 150 and 350° C., inparticular between 190 and 300° C.

The layer produced in accordance with step C) has a thickness of from 20to 4000 μm, preferably of from 30 to 3500 μm, in particular of from 50to 3000 μm.

The treatment of the membrane produced in accordance with step C) in thepresence of moisture at temperatures and for a period of time until thelayer exhibits a sufficient strength for use in fuel cells. Thetreatment can be effected to the extent that the membrane isself-supporting so that it can be detached from the support without anydamage.

The treatment of the membrane in step D) is performed at temperatures ofmore than 0° C. and less than 150° C., preferably at temperaturesbetween 10° C. and 120° C., in particular between room temperature (20°C.) and 90° C., in the presence of moisture or water and/or steam and/orwater-containing phosphoric acid of up to 85% and/or in a mixture of amixture containing organic phosphonic acids and/or sulphonic acids inwater or phosphoric acid. The treatment is preferably performed atnormal pressure, but can also be carried out with action of pressure. Itis essential that the treatment takes place in the presence ofsufficient moisture whereby the organic phosphonic anhydrides presentcontribute to the solidification of the membrane by means of partialhydrolysis with formation of organophosphonic acids and/or phosphoricacid (if polyphosphoric acid was also used).

The organophosphonic acids formed in the hydrolysis of the organicphosphonic anhydrides

result in an unexpected reduction in the overvoltage, in particular atthe cathode in the membrane electrode unit which is produced from themembrane according to the invention.

The partial hydrolysis of the organic phosphonic anhydrides in step D)leads to a solidification of the membrane and a reduction in the layerthickness and the formation of a membrane having a thickness between 15and 3000 μm, preferably between 20 and 2000 μm, in particular between 20and 1500 μm, which is self-supporting.

The upper temperature limit for the treatment in accordance with step D)is typically 150° C. With extremely short action of moisture, forexample from overheated steam, this steam can also be hotter than 150°C. The duration of the treatment is substantial for the upper limit ofthe temperature.

The partial hydrolysis (step D) can also take place in climatic chamberswhere the hydrolysis can be specifically controlled with definedmoisture action. In this connection, the moisture can be specificallyset via the temperature or saturation of the surrounding area in contactwith it, for example gases such as air, nitrogen, carbon dioxide orother suitable gases, or steam. The duration of the treatment depends onthe parameters chosen as aforesaid.

Furthermore, the duration of the treatment depends on the thickness ofthe membrane.

Typically, the duration of the treatment amounts to a few seconds tominutes, for example with action of overheated steam, or up to wholedays, for example in the open air at room temperature and lower relativehumidity. Preferably, the duration of the treatment is 10 seconds to 300hours, in particular 1 minute to 200 hours.

If the partial hydrolysis is performed at room temperature (20° C.) withambient air having a relative humidity of 40-80%, the duration of thetreatment is 1 to 200 hours.

The membrane obtained in accordance with step D) can be formed in such away that it is self-supporting, i.e. it can be detached from the supportwithout any damage and then directly processed further, if applicable.

The concentration of phosphonic acid and therefore the conductivity ofthe polymer membrane according to the invention can be set via thedegree of hydrolysis, i.e. the duration, temperature and ambienthumidity. According to the invention, the concentration of protons isgiven as the ion exchange capacity (IEC). Within the scope of thepresent invention, an IEC is at least 2 eq/g, preferably 5 eq/g,particularly preferably 10 eq/g.

Following the treatment in accordance with step D), the membrane canfurther be cross-linked at the surface by action of heat in the presenceof atmospheric oxygen. This curing of the membrane surface additionallyimproves the properties of the membrane.

The cross-linking can also take place by action of IR or NIR(IR=infrared, i.e. light having a wavelength of more than 700 nm;NIR=near-IR, i.e. light having a wavelength in the range of from about700 to 2000 nm and an energy in the range of from about 0.6 to 1.75 eV,respectively). Another method is β-ray irradiation. In this connection,the irradiation dose is from 5 and 200 kGy.

The polymer membrane according to the invention has improved materialproperties compared to the doped polymer membranes previously known. Inparticular, they exhibit better performances in comparison with knowndoped polymer membranes. The reason for this is in particular animproved proton conductivity. This is at least 0.1 S/cm, preferably atleast 0.11 S/cm, in particular at least 0.12 S/cm at temperatures of120° C.

In addition to the polymers based on polyazoles, the membranes accordingto the invention can also include further polymers as blend material. Inthis case, the function of the blend component is essentially to improvethe mechanical properties and reduce the cost of material.

To this end, the additional blend material can be added before, duringor after step A) and/or step B) or before step C). As the blendmaterial, polyethersulphones, in particular the polyethersulphonesdescribed in the German patent application No. 10052242.4, come intoconsideration. The further polymers which can be employed as the blendcomponent include, amongst others, polyolefines, such aspoly(chloroprene), polyacetylene, polyphenylene, poly(p-xylylene),polyarylmethylene, polyarmethylene, polystyrene, polymethylstyrene,polyvinyl alcohol, polyvinyl acetate, polyvinyl ether, polyvinyl amine,poly(N-vinyl acetamide), polyvinyl imidazole, polyvinyl carbazole,polyvinyl pyrrolidone, polyvinyl pyridine, polyvinyl chloride,polyvinylidene chloride, polytetrafluoroethylene,polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene,with perfluoropropylvinyl ether, with trifluoronitrosomethane, withsulphonyl fluoride vinyl ether, with carbalkoxyperfluoroalkoxyvinylether, polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidenefluoride, polyacrolein, polyacrylamide, polyacrylonitrile,polycyanoacrylates, polymethacrylimide, cycloolefinic copolymers, inparticular of norbornenes;

polymers having C-0 bonds in the backbone, for example

polyacetal, polyoxymethylene, polyether, polypropylene oxide,polyepichlorohydrin, polytetrahydrofuran, polyphenylene oxide, polyetherketone, polyester, in particular polyhydroxyacetic acid,polyethyleneterephthalate, polybutyleneterephthalate,polyhydroxybenzoate, polyhydroxypropionic acid, polypivalolacton,polycaprolacton, polymalonic acid, polycarbonate;polymeric C—S bonds in the backbone, for example polysulphide ether,polyphenylenesulphide, polyethersulphone;polymeric C—N bonds in the backbone, for example polyimines,polyisocyanides, polyetherimine, polyaniline, polyamides,polyhydrazides, polyurethanes, polyimides, polyazoles, polyazines;liquid crystalline polymers, in particular Vetra, as well asinorganic polymers, such as polysilanes, polycarbosilanes,polysiloxanes, polysilicic acid, polysilicates, silicons,polyphosphazenes and polythiazyl.

For the application in fuel cells with a long-term service temperatureabove 100° C., such blend polymers that have a glass transitiontemperature or Vicat softening point VST/A/50 of at least 100° C.,preferably at least 150° C. and especially particularly preferably atleast 180° C., are preferred. In this connection, polysulphones with aVicat softening point VST/A/50 of from 180° C. to 230° C. are preferred.

The preferred polymers include polysulphones, in particular polysulphonehaving aromatic groups in the backbone. According to a particular aspectof the present invention, preferred polysulphones and polyethersulphoneshave a melt volume rate MVR 300121.6 of less than or equal to 40 cm³/10min, in particular less than or equal to 30 cm³/10 min and particularlypreferably less than or equal to 20 cm³/10 min, measured in accordancewith ISO 1133.

According to a particular aspect, the polymer membrane can comprise atleast one polymer with aromatic sulphonic acid groups and/or phosphonicacid groups. Aromatic sulphonic acid groups and/or phosphonic acidgroups are groups in which the sulphonic acid groups (—SO₃H) and/orphosphonic acid groups (—PO₃H₂) are bound covalently to an aromatic orheteroaromatic group. The aromatic group can be part of the backbone ofthe polymer or part of a side group where polymers having aromaticgroups in the backbone are preferred. The sulphonic acid groups and/orphosphonic acid groups can in many cases also be employed in the form oftheir salts. Furthermore, derivatives, for example esters, in particularmethyl or ethyl esters, or halides of the sulphonic acids can be usedwhich are converted to the sulphonic acid during operation of themembrane.

Preferred aromatic or heteroaromatic groups are derived from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenylsulphone, thiophene, furan,pyrrole, thiazole, oxazole, imidazole, isothiazole, isoxazole, pyrazole,1,3,4-oxadiazole, 2,5-diphenyl-1,3,4-oxadiazole, 1,3,4-thiadiazole,1,3,4-triazole, 2,5-diphenyl-1,3,4-triazole,1,2,5-triphenyl-1,3,4-triazole, 1,2,4-oxadiazole, 1,2,4-thiadiazole,1,2,4-triazole, 1,2,3-triazole, 1,2,3,4-tetrazole, benzo[b]thiophene,benzo[b]furan, indole, benzo[c]thiophene, benzo[c]furan, isoindole,benzoxazole, benzothiazole, benzimidazole, benzisoxazole,benzisothiazole, benzopyrazole, benzothiadiazole, benzotriazole,dibenzofuran, dibenzothiophene, carbazole, pyridine, bipyridine,pyrazine, pyrazole, pyrimidine, pyridazine, 1,3,5-triazine,1,2,4-triazine, 1,2,4,5-triazine, tetrazine, quinoline, isoquinoline,quinoxaline, quinazoline, cinnoline, 1,8-naphthyridine,1,5-naphthyridine, 1,6-naphthyridine, 1,7-naphthyridine, phthalazine,pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine,diphenyl ether, anthracene, benzopyrrole, benzooxathiadiazole,benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine,benzopyrimidine, benzotriazine, indolizine, pyridopyridine,imidazopyrimidine, pyrazinopyrimidine, carbazole, aziridine, phenazine,benzoquinoline, phenoxazine, phenothiazine, acridizine, benzopteridine,phenanthroline and phenanthrene which optionally also can besubstituted. Preferred substituents are halogen atoms, e.g. fluorine,amino groups, hydroxy groups or alkyl groups.

In this case, the substitution pattern can be in any form, in the caseof phenylene, for example, it can be ortho-, meta- and para-phenylene.Particularly preferred groups are derived from benzene and biphenylenewhich optionally also can be substituted.

Preferred alkyl groups are short-chain alkyl groups having 1 to 4 carbonatoms, e.g. methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups can be substituted.

The polymers modified with sulphonic acid groups preferably have acontent of sulphonic acid groups in the range of from 0.5 to 3 meq/g,preferably 0.5 to 2 meq/g. This value is determined through theso-called ion exchange capacity (IEC).

To measure the IEC, the sulphonic acid groups are converted to the freeacid. To this end, the polymer is treated in a known way with acid,removing excess acid by washing. Thus, the sulphonated polymer isinitially treated for 2 hours in boiling water. Subsequently, excesswater is dabbed off and the sample is dried at 160° C. in a vacuumdrying cabinet at p<1 mbar for 15 hours. Then, the dry weight of themembrane is determined. The polymer thus dried is then dissolved in DMSOat 80° C. for 1 h. Subsequently, the solution is titrated with 0.1MNaOH. The ion exchange capacity (IEC) is then calculated from theconsumption of acid to reach the equivalence point and from the dryweight.

Polymers with sulphonic acid groups covalently bound to aromatic groupsare known in professional circles. Polymers with aromatic sulphonic acidgroups can, for example, be produced by sulphonation of polymers.Processes for the sulphonation of polymers are described in F. Kucera etal., Polymer Engineering and Science 1988, Vol. 38, No. 5, 783-792. Inthis connection, the sulphonation conditions can be chosen such that alow degree of sulphonation develops (DE-A-19959289).

With regard to polymers having aromatic sulphonic acid groups whosearomatic radicals are part of the side group, particular reference shallbe made to polystyrene derivatives. The document U.S. Pat. No. 6,110,616for instance describes copolymers of butadiene and styrene and theirsubsequent sulphonation for use in fuel cells.

Furthermore, such polymers can also be obtained by polyreactions ofmonomers which comprise acid groups. Thus, perfluorinated polymers asdescribed in U.S. Pat. No. 5,422,411 can be produced by copolymerisationof trifluorostyrene and sulphonyl-modified trifluorostyrene.

According to a particular aspect of the present invention,thermoplastics stable at high temperatures which include sulphonic acidgroups bound to aromatic groups are employed. In general, such polymershave aromatic groups in the backbone. Thus, sulphonated polyetherketones (DE-A-4219077, WO96/01177), sulphonated polysulphones (J. Membr.Sci. 83 (1993), p. 211) or sulphonated polyphenylenesulphide(DE-A-19527435) are preferred.

The polymers set forth above which have sulphonic acid groups bound toaromatic groups can be used individually or as a mixture where mixtureshaving polymers with aromatic groups in the backbone are particularlypreferred.

The molecular weight of the polymers having sulphonic acid groups boundto aromatic groups can vary widely, depending on the type of polymer andits processability. Preferably, the weight average of the molecularweight M_(w) is in the range of from 5000 to 10,000,000, in particular10,000 to 1,000,000, particularly preferably 15,000 to 50,000. Accordingto a particular aspect of the present invention, polymers with sulphonicacid groups bound to aromatic groups which have a low polydispersityindex M_(w)/M_(n) are. Preferably, the polydispersity index is in therange of from 1 to 5, in particular 1 to 4.

To improve the properties in terms of application technology further,fillers, in particular proton-conducting fillers, and additional acidscan additionally be added to the membrane. The addition can be performedeither before, during or after step A) and/or step B) or before step C).

Non-limiting examples of proton-conducting fillers are

-   sulphates, such as    -   CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄, KHSO₄, RbSO₄,        LiN₂H₅SO₄, NH₄HSO₄,-   phosphates, such as    -   Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄.3H₂O, H₈UO₂PO₄,        Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄, LiH₂PO₄, NH₄H₂PO₄,        CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄, H₅Sb₅P₂O₂₀,-   polyacid, such as    -   H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O (n=21-29), H_(x)WO₃,        HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃, HTiNbO₅, HTiTaO₅,        HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄-   selenides and arsenides, such as    -   (NH₄)₃H(SeO₄)₂, UO₂AsO₄, (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂,        Rb₃H(SeO₄)₂,-   phosphides, such as ZrP, TiP, HfP-   oxides, such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃-   silicates, such as zeolites, zeolites (NH₄+), phyllosilicates,    tectosilicates, H-natrolites, H-mordenites, NH₄-analcines,    NH₄-sodalites, NH₄-gallates, H-montmorillonites-   acids, such as HClO₄, SbF₅-   fillers, such as carbides, in particular SiC, Si₃N₄, fibres, in    particular glass fibres, glass powders and/or polymer fibres,    preferably based on polyazoles.

As a further component, this membrane can also contain perfluorinatedsulphonic acid additives (0.1-20 wt-%, preferably 0.2-15 wt-%,especially preferably 0.2-10 wt-%). These additives result in animprovement in performance, to an increase in oxygen solubility andoxygen diffusion in the vicinity of the cathode and to a reduction inadsorption of phosphoric acid and phosphate onto platinum. (Electrolyteadditives for phosphoric acid fuel cells. Gang, Xiao; Hjuler, H. A.;Olsen, C.; Berg, R. W.; Bjerrum, N. J. Chem. Dep. A, Tech. Univ.Denmark, Lyngby, Den. J. Electrochem. Soc. (1993), 140(4), 896-902, andPerfluorosulfonimide as an additive in phosphoric acid fuel cell. Razaq,M.; Razaq, A.; Yeager, E.; DesMarteau, Darryl D.; Singh, S. Case Cent.Electrochem. Sci., Case West. Reserve Univ., Cleveland, Ohio, USA. J.Electrochem. Soc. (1989), 136(2), 385-90.)

Non-limiting examples of persulphonated additives are:

trifluoromethanesulphonic acid, potassium trifluoromethanesulphonate,sodium trifluoromethanesulphonate, lithium trifluoromethanesulphonate,ammonium trifluoromethanesulphonate, potassiumperfluorohexanesulphonate, sodium perfluorohexanesulphonate, lithiumperfluorohexanesulphonate, ammonium perfluorohexanesulphonate,perfluorohexanesulphonic acid, potassium nonafluorobutanesulphonate,sodium nonafluorobutanesulphonate, lithium nonafluorobutanesulphonate,ammonium nonafluorobutanesulphonate, cesium nonafluorobutanesulphonate,triethylammonium perfluorohexasulphonate, perfluorosulphonimides andNafion.

As a further component, the membrane can also contain additives whichscavenge (primary antioxidants) or destroy (secondary antioxidants) thefree peroxide radicals produced in the oxygen reduction during operationand thereby improve the life and stability of the membrane and membraneelectrode unit as described in JP2001118591 A2. The functionality andmolecular structures of such additives are described in F. Gugumus inPlastics Additives, Hanser Verlag, 1990; N. S. Allen, M. EdgeFundamentals of Polymer Degradation and Stability, Elsevier, 1992; or H.Zweifel, Stabilization of Polymeric Materials, Springer, 1998.

Non-limiting examples of such additives are:

bis(trifluoromethyl) nitroxide, 2,2-diphenyl-1-picrinylhydrazyl,phenols, alkylphenols, sterically hindered alkylphenols, such as forexample Irganox, aromatic amines, sterically hindered amines, such asfor example Chimassorb; sterically hindered hydroxylamines, stericallyhindered alkylamines, sterically hindered hydroxylamines, stericallyhindered hydroxylamine ethers, phosphites, such as for example Irgafos,nitrosobenzene, methyl-2-nitrosopropane, benzophenone, benzaldehydetert-butyl nitrone, cysteamine, melanines, lead oxides, manganeseoxides, nickel oxides, cobalt oxides.

Possible fields of use for the doped polymer membranes according to theinvention include, amongst others, the use in fuel cells, electrolysis,capacitors and battery systems. Owing to their property profile, thedoped polymer membranes are preferably used in fuel cells.

The present invention also relates to a membrane electrode unit whichincludes at least one polymer membrane according to the invention. Forfurther information on membrane electrode units, reference is made tothe technical literature, in particular the U.S. Pat. No. 4,191,618,U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805. The disclosurecontained in the above-mentioned citations [U.S. Pat. No. 4,191,618,U.S. Pat. No. 4,212,714 und U.S. Pat. No. 4,333,805] with respect to thestructure and production of membrane electrode units as well as theelectrodes, gas diffusion layers and catalysts to be chosen is also partof the description.

In a variant of the present invention, the membrane formation can alsobe performed directly on the electrode rather than on a support. Throughthis, the treatment in accordance with step D) can be correspondinglyshortened since it is no longer required for the membrane to beself-supporting. Such a membrane is also an object of the presentinvention.

A further object of the present invention is an electrode having aproton-conducting polymer coating based on polyazoles, which can beobtained by a process comprising the steps of

-   A) dissolving the polyazol-polymer in organic phosphonic anhydrides    with formation of a solution and/or dispersion,-   B) heating the solution obtainable in accordance with step A) under    inert gas to temperatures of up to 400° C., preferably up to 350°    C., particularly of up to 300° C.,-   C) forming a membrane using the solution of the polyazole polymer in    accordance with step B) on an electrode and-   D) treatment of the membrane formed in step C) until it is    self-supporting.

The variants described above and the preferred embodiments also apply tothis object so that they will not be repeated at this point.

The coating has, following step D), a thickness between 2 and 3000 μm,preferably between 3 and 2000 μm, in particular between 5 and 1500 μm.

An electrode coated in such a way can be integrated into a membraneelectrode unit which optionally includes at least one polymer membraneaccording to the invention.

General Measurement Methods:

Measurement Method for IEC

The conductivity of the membranes depends strongly on the content ofacid groups, expressed as the so-called ion exchange capacity (IEC). Tomeasure the ion exchange capacity, a specimen having a diameter of 3 cmis punched out and placed in a beaker filled with 100 ml of water. Theacid released is titrated with 0.1M NaOH. Subsequently, the specimen isremoved, excess water is dabbed off and the sample is dried at 160° C.over 4 h. The dry weight, m₀, is then determined gravimetrically with anaccuracy of 0.1 mg. Thereafter, the ion exchange capacity is calculatedfrom the consumption of the 0.1M NaOH until reaching the first titrationendpoint, V₁ in ml, and from the dry weight, m₀ in mg, according to thefollowing formula:IEC=V ₁*300/m ₀Measurement Method for Specific Conductivity

The specific conductivity is measured by means of impedancy spectroscopyin a 4-pole arrangement in potentiostatic mode and using platinumelectrodes (wire, diameter of 0.25 mm). The distance between thecurrent-collecting electrodes is 2 cm. The spectrum obtained isevaluated using a simple model comprised of a parallel arrangement of anohmic resistance and a capacitor. The cross-section of the specimen ofthe membrane doped with phosphoric acid is measured immediately beforemounting the specimen. To measure the temperature dependency, themeasurement cell is brought to the desired temperature in an oven andregulated using a Pt-100 thermocouple arranged in the immediate vicinityof the specimen. After the temperature has been reached, the specimen iskept at this temperature for 10 minutes before beginning themeasurement.

The invention claimed is:
 1. A proton-conducting polymer membrane basedon polyazoles and further containing organic phosphonic anhydride, andpartial hydrolysis products of the organic phosphonic anhydride whichcan be obtained by a process comprising the steps of: A) dissolving thepolyazole polymer in organic phosphonic anhydrides with formation of asolution and/or dispersion, B) heating the solution obtainable inaccordance with step A) under inert gas to temperatures of up to 400°C., C) forming a membrane using the solution of the polyazole polymer inaccordance with step B) on a support and D) treating the membrane formedin step C) in the presence of moisture at temperatures and for a periodof time until the membrane is self-supporting and can be detached fromthe support without any damage and wherein in step A), said organicphosphonic anhydrides are of the formula (1), (2) or (3) and saidpartial hydrolysis products of the organic phosphonic anhydride are thepartial hydrolysis products of the organic phosphonic anhydride of theformulas (1), (2) or (3), wherein: formula (1) is of the formula

formula (2) is a linear compound of the formula

and formula (3) is an anhydride of the multiple organic phosphonic acidsof the formula

wherein the radicals R and R′ are identical or different and represent aC₁-C₂₀ carbon-containing group selected from the group consisting of:C₁-C₂₀ alkyl, C₆-C₂₀ aryl, C₁-C₂₀ fluoroalkyl, C₆-C₂₀ fluoroaryl, C₁-C₂₀alkoxy, C₆-C₂₀ aryloxy, C₇-C₂₀ arylalkyl, C₇-C₂₀ alkylaryl, C₇-C₂₀aryloxyalkyl, C₁₂-C₂₀ aryloxyaryl, C₇-C₂₀ heteroaryl, C₄-C₂₀heterocycloalkyl and C₂-C₂₀ heteroatom-containing group; and where oneor more of the C₁-C₂₀ carbon-containing groups can form a cyclic system.2. The membrane according to claim 1, wherein in step B) said heating isup to 300° C.
 3. The membrane according to claim 1, which furthercomprises in step A), a polyphosphoric acid having a content of at least83%, calculated as P₂O₅ by acidimetry.
 4. The membrane according toclaim 1, which further comprises in step A), P₂O₅.
 5. The membraneaccording to claim 1, wherein in step A), B) or step C), a solution or adispersion/suspension is produced.
 6. The membrane according to claim 1,wherein the polyazole polymer used in step A) contains recurring azoleunits of the general formula (I) and/or (II) wherein

Ar are identical or different and represent a tetracovalent aromatic orheteroaromatic group which can be mononuclear or polynuclear, Ar″ areidentical or different and represent a bicovalent aromatic orheteroaromatic group which can be mononuclear or polynuclear, Ar² areidentical or different and represent a bicovalent or bicovalent aromaticor heteroaromatic group which can be mononuclear or polynuclear, and Xare identical or different and represent oxygen, sulphur or an aminogroup which carries a hydrogen atom, or a group having 1-20 carbonatoms.
 7. The membrane according to claim 6, wherein X are identical ordifferent and represent oxygen, sulphur or an amino group which carriesa hydrogen atom, a branched or -unbranched alkyl or alkoxy group, or anaryl group as a further radical.
 8. The membrane according to claim 1,wherein, in step A), the polyazole polymer is selected from the groupconsisting of polybenzimidazole, poly(pyridines), poly(pyrimidines),polyimidazoles, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles,polyquinoxalines, polythiadiazoles and poly(tetrazapyrenes) is used. 9.The membrane according to claim 1, wherein, before, during or after stepA) and/or step B) or before step C), a further polymer is added as blendmaterial.
 10. The membrane according to claim 1, wherein, before stepD), the viscosity is adjusted by addition of phosphoric acid and/ororganophosphonic acids.
 11. The membrane according to claim 1, whereinthe treating of the membrane in step D) is performed at temperatures ofmore than 0° C. and less than 150° C., in the presence of moisture orwater and/or steam.
 12. The membrane according to claim 1, wherein thetreating of the membrane in step D) is performed at temperatures of morethan 20° C. and less than 90° C., in the presence of moisture or waterand/or steam.
 13. The membrane according to claim 1, wherein thetreating of the membrane in step D) is for 10 seconds to 300 hours. 14.The membrane according to claim 1, wherein, in step C), an electrode ischosen as the support and the treatment in accordance with step C) issuch that the membrane formed is no longer self-supporting.
 15. Themembrane according to claim 1, wherein, in step C), a layer having athickness of 20 to 4,000 μm, is produced.
 16. The membrane according toclaim 1, wherein the membrane formed in step D) has a thickness between15 and 3,000 μm.
 17. A membrane electrode unit containing at least oneelectrode and at least one membrane according to claim
 16. 18. Themembrane according to claim 1, wherein in step A), the polyazole polymerused contains one or more recurring benzimidazole units of the formula

where n is an integer greater than or equal to
 10. 19. The membraneaccording to claim 1, wherein the treating of the membrane in step D) isperformed at temperatures of more than 20° C. and less than 90° C., inthe presence of moisture or water and/or steam for 1 minute to 200 hoursand the membrane formed in step D) has a thickness between 20 and 1,500μm.
 20. The membrane according to claim 1, wherein the radicals R and R′are identical or different and represent methyl, ethyl, n-propyl,i-propyl, n-butyl, i-butyl, s-butyl, t-butyl, n-pentyl, s-pentyl,cyclopentyl, n-hexyl, cyclohexyl, n-octyl, cyclooctyl, phenyl, biphenyl,naphthyl, anthracenyl, trifluoromethyl, pentafluoroethyl,2,2,2-trifluoroethyl, triphenylenyl, [1,1′;3′,1]-terphenyl-2′-yl,binaphthyl, phenanthrenyl, tetrafluorophenyl, heptafluoronaphthyl,methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy,t-butoxy, phenoxy, naphthoxy, biphenyloxy, anthracenyloxy,phenanthrenyloxy, o-tolyl, m-tolyl, p-tolyl, 2,6-dimethylphenyl,2,6-diethylphenyl, 2,6-di-i-propylphenyl, 2,6-di-t-butylphenyl,o-t-butylphenyl, m-t-butylphenyl, p-t-butylphenyl, benzyl, ethylphenyl,propylphenyl, diphenylmethyl, triphenylmethyl, naphthalenyl methyl,o-methoxyphenyl, m-phenoxymethyl, p-phenoxymethyl, p-phenoxyphenyl,2-pyridyl, 3-pyridyl, 4-pyridyl, quinolinyl, isoquinolinyl, acridinyl,benzoquinolinyl, benzo-isoquinolinyl, furyl, benzofutyl, 2-pyrrolidinyl,2-indolyl, 3-indolyl, 2,3-dihydroindolyl, carbonyl, benzoyl, oxybenzoyl,benzoyloxy, acetyl, acetoxy or nitril.
 21. An electrode having aproton-conducting polymer coating based on polyazoles and furthercontaining organic phosphonic anhydride, and partial hydrolysis productsof the organic phosphonic anhydride which can be obtained by a processcomprising the steps of: A) dissolving the polyazole polymer in organicphosphonic anhydrides with formation of a solution and/or dispersion, B)heating the solution obtainable in accordance with step A) under inertgas to temperatures of up to 400° C., C) forming a membrane using thesolution of the polyazole polymer in accordance with step B) on anelectrode and D) treating the layer formed in step C) in the presence ofmoisture and wherein in step A), said organic phosphonic anhydrides areof the formula (1), (2) or (3) and said partial hydrolysis products ofthe organic phosphonic anhydride are the partial hydrolysis products ofthe organic phosphonic anhydride of the formulas (1), (2) or (3),wherein: formula (1) is of the formula

formula (2) is a linear compound of the formula

and formula (3) is an anhydride of the multiple organic phosphonic acidsof the formula

wherein the radicals R and R′ are identical or different and represent aC₁-C₂₀ carbon-containing group selected from the group consisting of:C₁-C₂₀ alkyl, C₆-C₂₀ aryl, C₁-C₂₀ fluoroalkyl, C₆-C₂₀ fluoroaryl, C₁-C₂₀alkoxy, C₆-C₂₀ aryloxy, C₇-C₂₀ arylalkyl, C₇-C₂₀ alkylaryl, C₇-C₂₀aryloxyalkyl, C₁₂-C₇₀ aryloxyaryl, C₇-C₂₀ heteroaryl, C₄-C₂₀heterocycloalkyl and C₂-C₂₀ heteroatom-containing group; and where oneor more of the C₁-C₂₀ carbon-containing groups can form a cyclic system.22. The electrode according to claim 21, where the coating has athickness between 2 and 3,000 μm.
 23. A membrane electrode unitcontaining at least one electrode according to claim
 21. 24. A fuel cellcontaining one or more membrane electrode units according to claim 23.